As communications and communication services have increased, it has become necessary to provide communication cables in larger and larger numbers. This is particularly true in office buildings where more and more communication services are being demanded. Typically, rather than rewire an existing building, it has been found more economical to provide the needed communication services by running the communication cables in plenums. In general, a plenum is defined as a compartment or chamber to which one or more air ducts are connected and which forms part of the air distribution system. Generally, in existing buildings, plenums are readily formed by providing drop ceilings, which is typically a return air plenum, in a facility being rewired. Another alternative is to create a plenum beneath a raised floor of a facility.
From the above it is readily understood why it would be very advantageous to utilize a wiring scheme within these fairly accessible places. However, because these plenums handle environmental air, there is considerable concern regarding the hazards resulting from a fire. This concern is addressed in the National Electrical Code by requiring that communications cables for use in plenums pass a stringent flame and smoke evaluation. Consequently, in the manufacture of communication cables, the fire resistance ratings which allow for installation within certain areas of a building are of significant importance.
Currently, communication cables for use in plenums must meet the requirements of the Underwriter's Laboratory Standard 910, which is a Test Method For Fire and Smoke Characteristics of Cables Used In Air-Handling Spaces. This well known test, which is performed in a modified Steiner Tunnel, is a severe test that can be passed by communication cables using certain premium materials for insulation such as low smoke materials, for example, fluorinated ethylenepropylene (FEP), ethylene trifluorochloroethylene, or polyvinylidene fluoride (PVDF). In general, cables using such insulation materials which meet this test are approximately three times more expensive than a lower rated cable designed for the same communication application. However, communication cables failing this test must be installed within conduit, thereby eliminating the benefits of an economical, easily relocatable cable scheme.
In general, the manufacture of communication cables are well known. For example, U.S. Pat. No. 4,423,589, issued to Hardin et al. on Jan. 3, 1984 discloses a method of manufacturing a communications cable by forming a plurality of wire units by advancing groups of twisted wire pairs through twisting stations. Further, U.S. Pat. No. 4,446,689 issued to Hardin et al. on May 8, 1984 relates to an apparatus for manufacturing a communications cable wherein disc frames are provided with aligned apertures in which faceplates are movably mounted. During operation, the faceplates are modulated in both frequency and amplitude.
Likewise, the current materials for use in communication cables are also well known. For example, U.S. Pat. No. 5,001,304 issued to Hardin et al. on Mar. 19, 1991 relates to a building riser cable having a core which includes twisted pairs of metal conductors, wherein the insulating covers are formed from a group of materials including polyolefin. It should be noted however, that the flame test used for riser cables is much less severe than the flame test used for plenum cables.
U.S. Pat. No. 5,024,506 issued to Hardin et al. on Jun. 18, 1991 discloses a plenum cable that includes non-halogenated plastic insulation materials. The insulating material about the metallic conductors is selected from the group consisting of a polyetherimide, and a silicone-polyimide copolymer, or a blend of the two. Similarly, in U.S. Pat. No. 5,074,640 issued to Hardin et al. on Dec. 24, 1991 a plenum cable is described that includes an insulator containing a polyetherimide and an additive system including an antioxidant/thermal stabilizer and a metal deactuator.
U.S. Pat. No. 4,500,748 issued to Klein on Feb. 19, 1985 relates to a flame retardant plenum cable wherein the insulation and the jacket are made from the same or different polymers to provide a reduced amount of halogens.
U.S. Pat. No. 4,605,818 issued to Arroyo et al. on Aug. 12, 1986 relates to a flame retardant plenum cable wherein the conductor insulation is a polyvinyl chloride plastic provided with a flame retardant, smoke suppressive sheath system.
U.S. Pat. No. 4,678,294 issued to Angeles on Aug. 18, 1987 relates to a fiber optic plenum cable. The optical fibers are provided with a buffer layer surrounded by a jacket. The cable is also provided with strength members for rigidity.
U.S. Pat. No 5,493,071 issued to Newmoyer on Feb. 20, 1996 relates to a plenum cable for communications use wherein at least one of the twisted pairs of the cable has a "non-plenum rated" insulation material while the remaining twisted pairs have a "plenum rated" insulation material. Newmoyer defines "plenum rated insulation" as "those materials that would allow a cable to pass standard industry plenum tests if it were used on all of the twisted pairs of electrical conductors of a cable" and "non-plenum rated insulation" is defined as "those materials that would significantly contribute to a cable failing standard industry plenum tests if it were used on all of the twisted pairs of electrical conductors of a cable." Newmoyer further discloses that the twisted pair having the shortest twist length have the non-plenum rated insulation.
It can thus be understood that much work has been dedicated to providing not only communication cables that meet certain safety requirements but meet electrical requirements as well. Nevertheless, the most common communication cable that is in widest use today includes a plurality of twisted pairs of electrical conductors each having an insulation of FEP, which is a very high temperature material and possesses those electrical characteristics, such as, low dielectric constant and dissipation factor, necessary to provide high quality communications cable performance. However, FEP is quite expensive and is frequently in short supply.
Consequently, the provision of a communication cable for use in plenums but has a reduced use of FEP is highly desired.